Bus bars have been used to connect lead wires (hereinafter, referred to as coils) wound around motor stator cores in view of assemble efficiency. For example, JP-A-2001-103700 and JP-A-2001-86695 describe that bus bars are used because of their high connection efficiency and that the bus bars are manufactured by stamping copper plates.
Referring to FIGS. 1 and 2, the conventional motor-wiring bus bar (hereinafter, simply referred to as a bus bar) will be briefly described. FIG. 1 shows the mechanical structure of a brushless DC motor. FIG. 2 shows the electrical circuit thereof.
The mechanical structure will first be described. Referring to FIG. 1, a stator 1000 includes stator cores 51-U, 51-V, 51-W, 52-U, 52-V, and 52-W corresponding to U-phase, V-phase, and W-phase, respectively, and coils 101-U, 101-V, 101-W, 102-U, 102-V, and 102-W wound around the respective cores. The connection of the coils will then be described. For example, for the U-phase, one ends of the coils 101-U and 102-U are connected to a ring-shaped U-phase bus bar 1-U and the other ends are connected to a ring-shaped N-phase bus bar 1-N. For the V-phase, one ends of the coils 101-V and 102-V are connected to a ring-shaped V-phase bus bar 1-V and the other ends are connected to a ring shaped N-phase bus bar 1-N. The coils of W-phase are also connected similarly. The connection between the coils and the bus bars in FIG. 1 shows electrical connection, in which specific connecting means are not generally electrical wires but engagement of bus bars with coils. Accordingly, since the connection with the ring-shaped bus bars is easier than that with electrical wires, it is useful in increasing assembly efficiency.
Referring now to FIG. 2, the electrical connection will be described. A three-phase current is generated by an inverter including a power source DC and switches SU, SV, SW, SX, SY, and SZ, which is fed through power lines YU, YV, and YW to the coils 101-U, 102-U, 101-V, 102-V, 101-W, and 102-W of the stator 1000. In this example, since the coils are in Y-connection, one ends of the coils are connected with the U-, V-, W-phase bus bars 1-U, 1-V, and 1-W in correspondence with the respective phases and all of the other ends of the coils are connected with the neutral-point bus bar 1-N.
The operation of a motor with the mechanical and electrical structure will be briefly described. A three-phase current is fed to the coils of the motor by the inverter including the DC power source and the switches. The current is fed to the power lines YU, YV, and YW, the bus bars, and the coils, which will be specifically described with the U-phase as an example. The U-phase current flows through the power line YU into a terminal 33-U of the U-phase bus bar 1-U, passes through a U-phase ring conductor 2-U and is fed from a terminal 31-U to the coil 101-U or from a terminal 32-U to the coil 102-U. The current flowing through the coils 101-U and 102-U flows into the bus bar 1-N and then flows from the terminals of the bus bar 1-N into the V-phase coils 101-V and 102-V and the W-phase coils 101-W and 102-W. The currents in the coils 101-V and 102-V join together through terminals 31-V and 32-V of the bus bar 1-V at the bus bar 1-V and returns from a terminal 33-V through the power line YV to the DC power source. Also the W-phase coil current passes through the bus bar 1-W and returns to the DC power source. The three-phase current flowing through the U-, V-, and W-phase coils generates a rotary magnetic field to rotate a rotor (not shown), thereby driving the motor.
As will be understood from the fact that the coil currents flow into the bus bars to join together and flow out therefrom, the bus bars are very important members to combine the currents.
The conventional motors using the ring-shaped bus bar, however, have a structural problem in achieving size reduction. Specifically, referring to FIG. 3, since the stack of ring-shaped bus bars 1-U, 1-V, 1-W, and 1-N is used, it is four times as thick as that of the bus bar (four bus bars of U-, V-, W-, and N-phase), having a problem in reducing the size of the motor.
Also the method for manufacturing the motor including the ring-shaped bus bars has the following problem: The ring conductors have been made by stamping, thus having the problem that the remaining of the stamped member is wasted.
The present invention has been made in view of the above problems. Accordingly, it is a first object of the invention to provide a motor including a motor-wiring bus bar suitable for size reduction and a method for manufacturing a motor including an economical motor-wiring bus bar which makes good use of the member.
When low torque ripple is required for permanent-magnet motors with conventional rectangular wave drive (120-degree energization system), distributed winding has been adopted for coils. In this case, the equation (1) holds from the relationship between a possible number of poles, 2p, of the motor including the distributed coils and the number of slots, SS/2 pm=integer  (1)where m is the number of phases of the motor.
Since a cogging torque is in inverse proportion to the least common multiple of the number of poles, 2p, and the number of slots, S, it produces the problem of significantly increasing the cogging torque.
In order to solve the above problems, a permanent-magnet motor is provided which is disclosed in, for example, JP-A-10-126982. The permanent-magnet motor achieves the reduction of the cogging torque by arranging the teeth such that the centers of the slot openings of the stator deviate from the positions at equal intervals. More specifically, the permanent-magnet motor includes annular permanent magnets constituting M magnetic poles (M is an even number) to construct a rotor and T teeth facing the permanent magnets arranged at irregular intervals, a winding groove (slot) between the teeth being wound with a coil, and the teeth being joined to a yoke to construct a stator. The teeth are arranged such that the centers of the slot openings of the stator deviate from the position of regular intervals T by a deviation of ±N×180/{C(T/2+1)}, where C is the least common multiple of the number M of the permanent magnets and the number T of the teeth and N is an integer from 1 to T/2.
However, with the permanent-magnet motor disclosed in the above-described JP-A-10-126982, the phases of voltage induced by the coils deviate, so that harmonic component of the voltage waveform induced between the coils are eliminated. In other words, it has the problem of increasing the torque ripple when the permanent magnetic motor is driven with a rectangular-wave current.
The present invention has been made in view of the above problems. Accordingly, it is a second object of the invention to provide a permanent-magnet motor capable of achieving low cogging torque, low torque ripple, and high output with compact size, being driven with a rectangular-wave current, and including a distributed-coil stator.
In an electrically driven power steering system that energizes an automotive steering system with motor torque as auxiliary, the driving force of the motor is applied to the steering shaft or the rack shaft through a decelerator and a transfer mechanism such as a gear and a belt to energize it with an auxiliary load. Referring to FIG. 4, the simple structure of the electrically driven power steering system will be described. A shaft 102 of a steering wheel 101 is joined to tie rods 106 of wheels through a reduction gear 103, universal joints 104a and 104b, and a pinion rack mechanism 105. The shaft 102 includes a torque sensor 107 for sensing the steering torque of the steering wheel 101. A motor 108 for applying an auxiliary power to the steering wheel 101 is joined to the shaft 102 through the reduction gear 103. It is important here that the motor 108 used for an important safety component such as the electrically driven power steering system is required to have significantly high reliability.
However, the motor 108 actually has disadvantages, one of which is that, with a brushless DC motor, foreign matter is caught between the rotor and the stator to lock the motor.
Referring now to FIG. 5, the mechanism of the motor lock will be described. FIG. 5(A) shows the longitudinal section of the brushless DC motor and FIG. 5(B) shows the cross section thereof. The brushless DC motor includes a rotor 9 having a permanent magnet 9-1 mounted to the outer periphery and a stator 10 wound with coils. The stator 10 includes a stator core 12 having a plurality of teeth 11 and coils 14 wound around slots 13 between the stator core 12 and the teeth 11. Since the brushless DC motor generally uses sintered permanent magnets for the rotor, the permanent magnets are covered with a nonmagnetic metal plate or the like to prevent locking even when the permanent magnets become chipped by external impact. The stator has openings between the adjacent teeth 11 to wind the coils 14 around the stator core 12. If foreign matter 50 such as insulator chips or varnish for hardening the coil enters the space between the rotor 9 and the teeth 11 through the openings, there is a possibility that the rotor is locked. An example of the method for preventing the motor lock is adopting a divided core in which the inner peripheries of the adjacent teeth are joined together. The joining of the teeth, however, increases leakage of flux to decrease the torque constant of the brushless DC motor, thus reducing output. Also, it rapidly increases the ripple of the counter electromotive voltage to increase torque ripple, thus decreasing the operability of the electrically driven power steering system. Another lock prevention method is molding the stator and the coil and filling the adjacent openings with resin, which has a disadvantage of increasing the cost.
As described above, the sudden locking of the brushless DC motor used as an important safety component such as the electrically driven power steering system causes dangerous car driving operation.
The invention has been made in view of the above problems. Accordingly, it is a third object of the invention to provide a brushless DC motor in which the motor output does not decrease, the torque ripple is low, the cost is low, and the motor is not locked.
The brushless motors used as the primary drive of automotive power steering systems generally include three or more exciting phases, which are driven with a rectangular-wave exciting current.
For example, with a five-phase brushless motor, the motor driving circuit rotates the rotor by exciting five-phase (hereinafter, referred to as an a-phase to e-phase) exciting coils a to e disposed to surround the outer periphery of the rotor of the motor at an interval of 72 electrical degrees with a rectangular-wave current, while switching the coils sequentially phase by phase, by four-phase simultaneous exciting under the control of a control circuit of a microcomputer, etc. With the four-phase simultaneous exciting, the motor current flows through four phases of the five phases; the circuit is constructed such that the resistances of the exciting coils are equal to feed the current through the phases in good balance.
The motor driving circuits are generally constructed of ten field effect transistors (FETs). The ten transistors connect to the coil circuit of the motor such that two corresponding transistors are connected in series to construct five series transistor circuits, each of which is connected between the positive and negative terminals of the power source, and in which the connections of the two transistors of each series transistor circuit are connected to the external ends of the five star (Y)-connected exciting coils a to e.
The direction and length of the exciting current (rectangular wave) fed from the motor driving circuit to the exciting coils relative to the rotation angle (electrical angle) of the rotor is shown in FIG. 6 by way of example. Specifically, the rotor is continuously rotated by sequentially switching the exciting coils phase by phase every 36 electrical degrees and exciting each phase coil during 144 electrical degrees. Referring to FIG. 6, the sections 0°≦θ<36°, 36°≦θ<72°, 72°≦θ<108°, 108°≦θ<144°, 144°≦θ<180°, 180°≦θ<216°, 216°≦θ<252°, 252°≦θ<288°, 288°≦θ<324°, and 324°≦θ<360° are indicated by (1) to (10), respectively, where θ is the electrical angle.
In this example, the a-phase current flows in the plus direction in sections (1) and (2), at zero in section (3), in the minus direction in sections (4) to (7), at zero in section (8), passes through sections (9) to (10), and again flows in the plus direction in section (1). The b-phase current flows in the plus direction in sections (1) to (4), at zero in section (5), in the minus direction in sections (6) to (9), at zero in section (10), and again flows in the plus direction in section (1). The c-phase current flows in the minus direction in section (1), at zero in section (2), in the plus direction in sections (3) to (6), at zero in section (7), passes through sections (8) to (10), and again flows in the minus direction in section (1). The d-phase current flows in the minus direction in sections (1) to (3), at zero in section (4), in the plus direction in sections (5) to (8), at zero in section (9) and again flows in the minus direction in section (10). The e-phase current flows at zero in section (1), in the minus direction in sections (2) to (5), at zero in section (6), in the plus direction in sections (7) to (10), and again becomes zero in section (1). Briefly, two of the five exciting coils are switched in the opposite direction at the boundaries (at the switching at every 36 electrical degrees) of the sections (1) to (10).
A brushless DC motor (BLDCM) and a permanent-magnet synchronous motor (PMSM) are known as the brushless motor. Examples of the former are disclosed in JP-A-11-356083 and JP-A-2002-269569 which adopt commutation control. An example of the latter is disclosed in JP-A-2000-201461.
Referring to FIG. 7, the brushless DC motor is lower in rpm in a low-torque load range than the permanent-magnet synchronous motor. However, it is possible to increase the rpm by weakening field control in the low-torque load range. However, this disadvantageously increases the torque ripple with an increased rpm to cause a noise due to the torque ripple.
With the permanent-magnet synchronous motor, the weakening field control facilitates the increase in rpm in the low-torque load range; however, decreases the rpm in a rated load range, being inferior to the brushless DC motor in terms of size reduction and high output of the motor in the rated load range.
The electrically driven power steering system requires a compact, high-output, low-torque-ripple, low-noise motor and controller and also high rpm during low torque loading (for example, in emergency).
The invention has been made in view of the above problems. Accordingly, it is a fourth object of the invention to provide a brushless-motor drive controller for a brushless DC motor in which high rpm is achieved and torque ripple is reduced during low torque loading.